This study is aimed at determining whether respiration plays a role in the initiation of sleep. The two overall objectives are first, to characterize changes in alveolar carbon dioxide tension as a parameter of respiration during relaxation and sleep onset in human subjects; and second, to determine whether voluntarily controlled changes in alveolar CO2 tension facilitated by biofeedback techniques can influence levels of arousal and sleep. METHODS: 1) Biofeedback training in deep relaxation will be used to separate out respiratory parameters of relaxation from those of sleep onset per se. Changes in alveolar CO2 tension and respiratory pattern will be correlated with EEG pattern in control and relaxation trained subjects. 2) Baseline recordings and recordings during alveolar CO2 biofeedback will be compared as to CO2 tension, respiratory pattern, and sleep onset latency as reflected in the EEG. This project assimilates for the first time a number of variables which have been linked to sleep onset and provides a unique approach to clarifying the observational data which has been gathered on the sleep onset process.